(1) Field of the Invention
This invention relates to an active matrix organic light emitting diode (AMOLED) display, a pixel driving circuit, and a driving method thereof, and more particularly to a voltage-driven AMOLED display.
(2) Description of the Related Art
With the progress in the fabrication technology of organic light emitting diodes (OELDs), an organic light emitting display with a plurality of OLEDs arranged in matrix has become a popular choice among all the flat panel displays. Based on different driving methods, the organic light emitting display can be sorted into simple matrix system type and active matrix system type. In addition, the active matrix system type is more suitable for large size displays and high resolution usage.
FIG. 1 shows a circuit diagram of a pixel driving circuit in a traditional voltage-driven active matrix organic light emitting display. The pixel driving circuit includes an OLED, a transistor T1, a transistor T2, and a capacitor C. A source electrode of the transistor T1 is connected to a data line D1 for receiving a driving voltage signal VD. A gate electrode of the transistor T1 is connected to a scan line S1. A source electrode of the transistor T2 is connected to an anode of the OLED. A drain electrode of the transistor T2 is provided with a potential Vdd. A gate electrode of the transistor T2 is connected to a drain electrode of the transistor T1. A cathode of the OLED is provided with a different potential Vss. Both ends of the capacitor C are connected to the gate electrode of the transistor T2 and provided with the potential Vdd respectively.
As to generate a steady current I passing through the OLED to maintain the brightness, a scanning voltage VS is firstly applied through the scan line S1 to turn on the transistor T1. Then, a driving voltage signal VD on the data line D1 is able to apply to the gate electrode of the transistor T2 and create a potential Vcs stored in the capacitor C. It is understood that the potential Vcs equals to a difference of the voltage levels of Vdd and the driving voltage signal VD. Therefore, the gate to source voltage Vgs (not shown) of the transistor T2 is determined. Since a difference between the gate to source voltage Vgs and the threshold voltage Vt of the transistor T2 determines the value of current I, the brightness of the OLED may be decided by setting the value of the driving voltage signal VD.
Although the usage of amorphous silicon thin film transistor (a-Si TFT) can reduce the cost of an organic light emitting display, most of the thin film transistors (TFT) applied for driving OLEDs nowadays are made of low temperature poly-silicon (LTPS) technology due to a major consideration of a shifting threshold voltage Vt of an a-Si TFT during operation. That is, even the gate to source voltage Vgs of the transistor remains constant, the value of the current passing through the OLED may be reduced due to the increasing threshold voltage Vt, and a decreasing brightness of the OLED is predictable.
The total variation of the threshold voltage Vt of the a-Si TFT is equal to a sum of the variations under positive bias and negative bias, which is disclosed in “Threshold Voltage Variation of Amorphous Silicon Thin-Film Transistor During Pulse Operation” of Japanese Journal of Applied Physics Vol. 30, December, 1991, pp. 3719-3723. Furthermore, the variation of the threshold voltage under positive bias is positive, and the variation of the threshold voltage under negative bias is negative, which is disclosed in “Electrical Instability of Hydrogenated Amorphous Silicon Thin-Film Transistors for Active-Matrix Liquid-Crystal Displays” of Japanese Journal of Applied Physics Vol. 37, September, 1998, pp. 4704-4710.
As mentioned above, the problem of increasing threshold voltage of the a-Si TFT can be effectively resolved by having the a-Si TFT properly supplied with negative bias. Therefore, modifying the pixel driving circuit by providing the TFT with negative bias is quite helpful for the application of a-Si TFT for driving OLEDs.